Compared to other semiconductor power switching devices like MOSFETs and IGBTs, the thyristor is generally known for its ability to sustain large current and its ability to be switched at high voltage. With a low on-state voltage drop for a given current density, a thyristor provides one of the lowest power dissipations among power semiconductor devices. A thyristor typically has four basic semiconductor layers, the emitter, base, drift and anode layers, respectively, with an alternative doping profile to form three junctions. Adjacent layers are oppositely doped with high doping on two outer layers and light doping on inner layers. When a voltage is applied between the anode electrode and cathode electrode, at least one junction is reverse biased to sustain a majority of the applied voltage, and the voltage is held mainly across the lightly doped drift layer until its breakdown.
A thyristor can be viewed as a pair of back-to-back coupled bipolar junction transistors. The lightly doped inner regions act as the base of two transistors. When a thyristor is under high voltage without protection, the leakage current across the lightly doped regions serves as the base current of the transistors and is amplified. When there are enough carriers inside the inner layers, the device is turned on. The first method of turning on a thyristor is, in fact, to apply a voltage higher than its blocking value. When the applied voltage is high enough, the thyristor is turned on through the current gain of the leakage current. However, the turn-on voltage is not precisely controllable and high voltages sometimes induce damage inside the device through breakdowns. In practice, the leakage current diverting structures like the cathode short and the anode short are added in the design to enhance the voltage holding capability.
For more controllable turn-on switching, the carriers are injected from a gate electrode on one of the inner layers. Usually, the gate current is only injected over a portion of the base layer so the conducting current of a thyristor is not fully spread over the whole layer initially. The thyristor will not be fully turned on until carriers spread across the layer by lateral diffusion. The time of carrier spreading depends on the lateral diffusion velocity, which limits the turn-on time. One way to circumvent the slow turn-on time is to inject the gate current over a large area. This may, in practice be implemented with inter-digitization of the gate electrodes and reduces the active cathode area for supporting high current.
An alternative approach for turning on a thyristor is to generate carriers locally inside the inner junctions through absorption of light. There have been several attempts in the prior art to use a photonic gate over the cathode area which permits light to pass through. Photo-generated carriers acting as the gate current injection in the base region start the turn-on process. With appropriate selection of the light wavelength, the depth of the light absorption across the device can be varied to fit different junction depths. Furthermore, a high level of illumination across the whole thyristor structure can instantaneously generate a high density of carriers across the whole device. The high density of carriers collapses the junction voltage and generates the current flow instantly without much delay from carriers being transported through the thyristor drift layer and the lateral diffusion from the gate electrode area to the main cathode area. Therefore, a light controlled thyristor has the advantages of shorter turn-on delay time and shorter turn-on time.
In practice, thyristors are employed in power circuits with high voltages and large currents. The trigger circuit to switch a thyristor through the gate electrode is difficult to isolate from high voltages. Instead of triggering through an electrical gate current, a high degree of electrical isolation between the power and trigger circuits may be achieved by switching with light through optical wave guide-like fibers.
Power MOSFETs and IGBTs will be switched off if the gates are turned off. Due to the self-sustaining effect of a thyristor, the current conduction of a thyristor will continue even after the gate is turned off. A thyristor does not need to maintain the gate injection like other power semiconductor devices. However, the gate of a thyristor loses control after the thyristor is switched on. To actively switch a thyristor from its on-state to the forward-blocking state can only be accomplished by reducing its current below a threshold or by reversal of the anode voltage. In an AC circuit, a thyristor is switched on and off in a cycle while the polarity of the voltage is alternative across the anode and cathode electrodes. However, it is not practical to switch polarity to reverse the anode voltage of a power device in many applications. Typically, the thyristor current is drained through a reverse gate current during turn-off.
In early attempts, the Gate Turn-Off thyristor (GTO) utilized an external control circuit to reverse the gate current and the MOS Controlled Thyristor (MCT) incorporates parallel MOSFETs to create emitter shorts. The external control circuit of a GTO diverts the current through the gate electrode and needs to carry a similar amount of current as a thyristor in order to switch off the thyristor. The diverted current is much larger than the turn-on gate current and this increases the difficulty of the control circuit design. Typically, the external current-diverting circuit of a GTO is much bigger in order to accommodate large thyristor current and there is basically no electrical isolation between the power and trigger circuits. On the other hand, a MCT uses MOSFETs to short the emitter and the base of a thyristor. Like the case of the external turn-off circuit of a GTO, MOSFETs in a MCT also need to take on the majority of the thyristor current. However, the current carrying capability of a MOSFET is limited by its surface channel and is much smaller than the bulk of a thyristor. Therefore, massively parallel MOSFET unit cells are integrated in order to carry large current. The MOSFETs occupy large real estates and limit the main conduction area of a MCT. Hence, MCTs have not been widely used for practical applications.
The limitation of the electronic switching-off of a thyristor is either due to the current carrying capability of MOSFETs to create emitter shorts or the limited current and speed of external current-diverting circuits. In addition, the electronic on-activation of a thyristor suffers slow turn-on and long delay due to the limited speed of carrier transport. A new control technique is needed to improve these short falls.
In earlier attempts, a photonic gate was employed in the light controlled thyristor to permit light. The illumination of light through the photonic gate generates carriers in the base region as the gate current injection to turn on the device. However, an external circuit is still employed for turning off a light controlled thyristor to divert the conducting current as a GTO. Various attempts with alternative electronic switching-off schemes also suffer similar short falls. Hence, the light controlled thyristor did not solve the whole problem that limits thyristors in many applications.
To improve the turn-off limitation of the light controlled thyristor, a photonic controllable switching structure was introduced on a thyristor. In O. S. F. Zucker et. al. (U.S. Pat. No. 6,218,682 B1, Apr. 17, 2001), the optically activated thyristor adds an external shorting structure on top of a light activated thyristor. The shorting structure is electrically and mechanically bonded across the emitter and base region of a thyristor. The added shorting structure comprises a PN junction and has an optical aperture for introducing light. Furthermore, an aperture over the emitter region for permitting light is introduced to better utilize the wafer surface area for current conduction instead of separated photonic gates. Therefore, a high level of light illumination may be introduced through the aperture to generate high density carriers in the bulk of the thyristor to direct short the whole device for fast switching on. During the conducting state of the thyristor, the shorting PN junction is open and under the back bias. When light is introduced onto the shorting structure, the photo-generated carriers collapse the voltage and electrically short the cathode and the base of the thyristor to create emitter shorts. The illuminated shorting structure diverts the conducting current to bypass the emitter and then turns off the thyristor. However, the main thyristor structure and the shorting structure are fabricated separately on different semiconductor wafers. The wafer with the shorting structure is then diced and externally bonded on the main thyristor structure. In addition to the alignment of the optical fibers to apertures, additional alignment and bonding of the shorting structure to the main thyristor structure in the back end processing increase the complexity and cost of the fabrication.
Accordingly, there remains a need for an improved light activated thyristor. There remains a further need for a thyristor that is compact and monolithically integrated, so that the complexity and cost of fabrication may be greatly reduced to fit practical applications.